you need to assign the file extension to notepad++ application by either using
the npp menu settings->preferences->file associations and use one of the predefined
extensions (if applicable) or define a custom one. Or right click on the file and selectopen with, select npp and check the box always open …

I’m not good at English, so I’ll write simply. I hope you understand me.
I used two way that you said.
First, I right-clicked on the file, selected ‘open with select notepad++’ and checked the box always open.
Next I opened the file was saved my data using notepad++, new note was opened instead of my file.(My file wasn’t opened.)
And even though I have set notepad ++ to open a txt file, when I double-clicked my file, file was opened with default notepad.
So I clicked the menu settings -> preferences -> file association -> supported extensions and registered extensions (.txt and .log).
But still new note was opened instead of my file.

I confirmed it according to your advice, I used npp 32bit.
My computer system was operated 64bit. So I installed npp 64bit from the official website, my file was opened when I clicked ‘edit with notepad++’ button.
However, after registering the Notepad extension (.txt, .log), the file extension was disappeared when I re-run npp.
I always have run as administrator using my work pc, here is debug output.

I can’t remember if this is normal or not and as I currently do not use
a windows operating system I cannot test this.
Do the files open with npp if you double click in explorer?
If yes, everything seems to be ok.
If not, give it a try to register extension by using right-click and check the box open always…

In regards to the 32bit vs. 64bit npp version.
You can use both versions even on a 64 bit Windows 7.
Each version has its advantage and its disadvantage.

Using the 64bit version eliminates the problem with file system redirection
but disadvantage is that, currently, not many plugins are available.